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Quantum Bell Experiments
Quantum Bell Experiments (QBE's) are a class of experiments in quantum history popularised by John Stewart Bell with his famous 'Bell's Theorem'. Bell's theorem recognised that the local wavefunctions of entangled particles are insufficient for predicting the outcomes of correlation measurements made to the joint-system in its super-local end state. Einstein concluded that such 'EPR-signalling' between particles would imply that information was travelling either faster than light, or backwards in time , both of which are mathematically impossible under the laws of general relativity. Instead, Einstein concluded (correctly) that (locally causal time-asymmetric) quantum physics was incomplete. Bell Test A Bell test essentially creates a scenario in which counterfactual evidence from multiple experiments imply limits to the possible results the experimenter can expect to find in the remaining experiments. Counterfactual evidence relies upon "if X then Y" reasoning, even in cases where "not X" is true. (MORE SOON) Bell's Inequality ... phrasing it as... Loophole-Free Bell Tests While Bell Tests from 1972https://en.wikipedia.org/wiki/Bell_test_experiments#Notable_experiments have shown that quantum statistics accurately predict the correlations in measurements of entangled particles, despite the freedom of choice of measurement basis, there are certain 'loopholes' in experiments that can be argued to invalidate the conclusion that quantum states are not ontologically-defined prior to measurement. Background Under the presumption that quantum states are defined prior to measurement (Psi-ontological theories) changing the measurement basis prior to measurement can not possibly change the pre-defined state of the quantum particles being measured. Yet, Bell Tests show that entangled quantum states are predicted to correlate regardless of the measurement basis. For example, if two states are 'anti-aligned' such that their quantum state-vectors are orthogonal (e.g. when one is 'spin-up', the other is 'spin-down') then measuring them in the basis that aligns with their existing states (e.g. the up vs down basis) will clearly give anti-correlated results (if you measure one as up, then other will NOT be measured as up). However, if you choose to measure the same two states in a basis that is not aligned to their supposed pre-defined states (e.g. measuring them in a 'spin-east' vs 'spin-west' basis) then they should have reduced correlation in this basis. The results of Bell Tests contradict this assumption and are hence implied to disprove ontological theories of quantum mechanics in that they negate the assumption that entangled states of multiple particles can be considered to have a individual physical states (though unknown) prior to measurement. Examples https://en.wikipedia.org/wiki/Bell_test_experiments#Notable_experiments |PRL:/Li2018/Test of Local Realism into the Past without Detection and Locality Loopholes> (|arXiv:/Li2018>) :"Inspired by the recent remarkable progress in the experimental test of local realism, we report here such a test that achieves an efficiency greater than (78%)^2 for entangled photon pairs separated by 183 m. Further utilizing the randomness in cosmic photons from pairs of stars on the opposite sides of the sky for the measurement setting choices, we not only close the locality and detection loopholes simultaneously, but also test the null hypothesis against local hidden variable mechanisms for events that took place 11 years ago (13 orders of magnitude longer than previous experiments). After considering the bias in measurement setting choices, we obtain an upper bound on the p value of 7.87 × 10^-4 , which clearly indicates the rejection with high confidence of potential local hidden variable models. One may further push the time constraint on local hidden variable mechanisms deep into the cosmic history by taking advantage of the randomness in photon emissions from quasars with large aperture telescopes." :"Noting that we have achieved high single-photon detection efficiency and ensured spacelike separation between relevant events, we now illustrate that we indeed close loopholes in our Bell test experiment, particularly the detection loophole, which has two related issues. One is related to the loss of entangled photons from creation to detection due to the system imperfection, and the other is related to the inefficiency in detecting cosmic photons. From a pedagogical perspective, we present the discussion with a general nonlocal game, where the two stars can be regarded as two referees. In each trial of the game, Alice and Bob, as two players who are not allowed to communicate during the trial, each receives a 1-bit random number as the input x and y, and outputs a 1-bit outcome a and b, respectively. The score for each trial is calculated according to the inputs and outputs. We stress that in each trial of the game, both referees give random bits, and the detection loophole problem arises when one or both players do not always have outputs (say, due to channel loss). In this case, Alice and Bob can prepare some (input and output) bits ahead and, if their input bits match the referees, they would output the corresponding output bits, and they would not provide outputs if the input bits are not matched. Such detection loophole was well studied in the past, which can be closed with high single-photon detection efficiency. On the other hand, it is okey that one or both referees sometimes do not want to play and therefore do not provide random bits in the game. We stress that it is not counted as a game trial when this happens; and it is counted as a game trial if and only if both referees provide random numbers at both sides. Hence, such cases do not introduce the detection loophole." Note: they do not close the 'free-will loophole' which is essentially always present, that questions whether the random events could somehow 'psychically collude' to produce correlations that align with the measurement basis prior to it being chosen, implying that the 'random' choice of measurement basis was actually predetermined from before the experiment even begun - (see Absolute Determinism) References Category:Quantum Category:Quantum Entanglement